Both public and private conventional telephone exchanges are normally powered with direct voltage. Alternating voltage from the mains supply is rectified and the direct voltage utilized for charging batteries. The voltage to the exchange is taken from these batteries, possibly after DC/DC conversion.
The operation of the exchange is thus ensured for a certain time during an interruption in the AC distribution. Such an arrangement also makes the exchange insensitive to transient-type disturbances coming from the mains supply, e.g., those caused by rapid load variations.